Preheat the oven to 350 degrees . In a bowl, combine 3/4 cup brown sugar, the walnuts, raisins, 1/4 cup melted butter and the cinnamon.
Grease a 9-by-13-inch baking pan. In a large bowl, combine the remaining 1-1/4 cups brown sugar, the flour, baking powder, baking soda and salt. In another bowl, combine the remaining melted butter, the milk, yogurt, eggs and vanilla. Mix into the flour mixture just enough to moisten. Transfer to the prepared pan and bake for 25 minutes. Top with the raisin-walnut mixture and bake for 5 to 10 minutes longer. Let cool slightly before slicing.